1. Field of the Invention
The present invention relates to an X-ray CT apparatus for irradiating an X-ray to a subject and collecting transmission data of the X-ray, to obtain an image about an internal structure of the subject from the collected data.
2. Description of the Related Art
Today, the most representative radiodiagnostic apparatuses include X-ray CT (Computed Tomography) apparatuses. The X-ray CT apparatuses are classified into various forms according to the collection scheme of projection data.
There is an R-R scheme (rotate-rotate or third generation scheme) as one of the data collection forms. The R-R scheme is a scheme oppositely arranged with an X-ray tube and an X-ray detector in a state spatially sandwiching a subject, to perform data collection while rotating the X-ray tube and the X-ray detector in unison round the subject. Namely, when the X-ray tube and the X-ray detector are rotated in unison round the subject, projection data is gathered in views at a constant angular interval (at an interval of sampling points).
The R-R-schemed X-ray CT apparatus is advantageous over the other schemes of apparatuses, in terms of scattered-ray removal capability, economy and so on. For this reason, the most of the X-ray CT apparatuses currently in service employ the R-R scheme.
However, there are technical difficulties unique to the R-R scheme. For example, sampling pitch (ray interval) is fixed by an arrangement pitch of the detector elements, thus disabling free control. Consequently, where sampling is not sufficiently fine, the problem of aliasing takes place. There is a practical difficulty in detector element arrangement with sufficient density because incurring the problems of lowered X-ray detection efficiency resulting from the interval of detector elements (dead zone) and of cost. For this reason, the R-R-schemed X-ray CT apparatus can be considered always exposed to the danger of artifact due to aliasing.
As a measure against this, a Q-Q (Quarter-Quarter) offset method (called Quarter-offset method, offset detector or the like) is employed as described in JP-A-53-126892 (Japanese Patent Application No. Sho-52-41666) and a paper “Peters T M and Lewitt R M: Computed Tomography with Fan Beam Geometry. J Comput Assist Tomogr, Vol. 1, No. 4, 1977, 429-436”.
According to the Q-Q offset method, the X-ray tube and the detector are oppositely arranged such that the ray closest to a rotation center upon rotation of the X-ray tube and the X-ray detector deviates (offsets) by a quarter of sampling pitch (pitch projected of a detector element pitch onto a vicinity of the rotation center) Δ from the rotation center. Due to this, the ray on each view is to sneak through between the rays of the immediately opposite views, thus enabling to effectively reduce the sampling pitch down to a half (see FIG. 9A in the later).
However, it is not practically easy to arrange the X-ray tube and the X-ray detector in a manner correctly securing the quarter-pitched offset amount based on the Q-Q offset method.
One of the reasons is because there is variation in the positional relationship between an X-ray tube housing and a focal point. During manufacture, alignment is carefully done on each X-ray tube. However, this is impossible to a perfect. Another reason lies in that the most of the X-ray CT apparatuses use an X-ray tube having two focuses large and small in size. In this case, generally the two focuses cannot be placed at the same point, i.e., usually several millimeters of positional error exists between the both focuses. Accordingly, alignment is done at one focus while the other focus (mostly, large focus) deviated from the QQ state (i.e., state correctly secured with a quarter-pitched offset amount based on the Q-Q offset method) is to be compromisingly used in scanning with thick slicing, e.g., 5 mm and 10 mm. With thick slicing, partial volume effect acts in the Z-axis direction and suppresses the high-frequency component of projection data, making aliasing not so conspicuous.
However, the problem of aliasing is in a tendency toward re-actualization due to the recent spread of multi-slice CT. There is currently spread of multi-slice CTs with 8 and 16 rows. For such a CT, scanning by thin slicing is quite common. Moreover, in order to supplement photon deficiency in thin slicing, there is a tendency of carrying out a scanning with the large focus even at a slicing of as small as 1-2 mm, thereby securing a dose. In this case, because such a thin slice is scanned in a state departed from the QQ state, there arises again a problem of artifact caused by aliasing on the reconstructed image.
In this manner, where adopting the Q-Q offset method, the problem of aliasing occurrence is again actualized due to the recent spread of multi-slice CTs. Thus, there is an urgent need for resolving the problem of aliasing caused on the R-R scheme by means of another approach than the Q-Q offset method.